wgengine: define default tunname for each platform

The device name "tailscale0" will be used for all platforms except for
OpenBSD where "tun" is enforced by the kernel. `CreateTUN()` in
`wireguard-go` will select the next available "tunX" device name on the
OpenBSD system.

Signed-off-by: Martin Baillie <martin@baillie.email>

wgengine/router_default.go

@@ -12,6 +12,8 @@ import (
 	"tailscale.com/types/logger"
 )
 
+const DefaultTunName = "tailscale0"
+
 func newUserspaceRouter(logf logger.Logf, tunname string, dev *device.Device, tuntap tun.Device, netChanged func()) Router {
 	return NewFakeRouter(logf, tunname, dev, tuntap, netChanged)
 }